Laser radar, also referred to as Light Detection and Ranging (LIDAR) or Laser Detection and Ranging (LADAR), is an active imaging technique which utilizes a laser in a radar system configuration to remotely image a scene and/or range to a target. Laser radar systems utilize principles of optics and microwave radar. Conventional laser radar systems are able to measure the shape, position and velocity of objects in a scene.
Conventional laser radar systems may be broadly divided into two categories: scanning and scannerless. Typical scanning laser radar systems include a laser, scanning optics, a timing system, a light detector system and a processor. To image a target scene, a typical scanning laser radar system first transmits a short pulse of light toward a point in the scene (target). The pulse of light may have a duration of approximately one nanosecond. Next, the detector system detects light reflected back from the point in the scene (target) and the timing system determines the round-trip travel time of the pulse of light.
The “round-trip travel time” of a pulse of light may be defined as the amount of time between the time that the laser transmits the pulse of light and the time that the detector system detects the reflected light. Next, the processor of the scanning laser system records the direction of the output of the laser and the round-trip travel time of the pulse of light. The scanning optics then position the output of the laser toward a new point in the scene and the laser radar system transmits a second pulse. This process is repeated for each point in the target scene. Finally, the processor generates an image of the scene in response to the recorded directions of the output of the laser and the corresponding round-trip travel times of each of the transmitted pulses of light.
Typical scannerless laser radar systems include a laser, a timing system, a stationary detector system and a processor. The detector system may include an array of light detectors. To image a target scene, a scannerless laser radar system directs the output of the laser toward the target scene and the laser transmits a pulse of light toward the target scene which illuminates the entire scene. Next, the detector system detects light reflected back from the scene. The timing system then determines a roundtrip travel time of the pulse of light for each of the light detectors in the array that detects the reflected light. Next, the processor records the positions of the light detectors in the array that detected the reflected light and the corresponding round-trip travel times of the pulse of light for each light detector. Finally, the processor determines an image of the target scene in response to the recorded positions of the light detectors and corresponding round-trip travel times of the transmitted pulse of light.
Generally, for both scanning and scannerless laser radar systems, range determination is made at low signal return levels. Multiple pulses must be processed to obtain an accurate range determination. Multiple pulses are needed to increase the signal to noise ratio of the range measurement, and thus the precision of the measurement. If integration time is critical, and a lesser number of pulses are desired to obtain the range measurement, then the laser radar system must use higher power transmitted pulses. This presents a disadvantage for a laser radar system, since cost of the system goes up with use of a higher power transmitter and higher power components.
Avalanche photodiodes (APDs) are photosensitive devices used to convert optical signals into electrical signals. As such, APDs behave like standard photodiodes, as both APDs and photodiodes convert optical energy into electrical signal. APDs, however, additionally incorporate a gain mechanism internal to the device itself, making it more sensitive. That is, in a conventional p-i-n photodiode an individual photon is converted into one electron-hole pair. In an APD, for each individual photon absorbed, however, multiple electron-hole pairs are generated. This multiplication, however, introduces unwanted noise to the APD's output.
APDs may be operated in two regimes: a linear regime and a breakdown regime, the latter often referred to as the Geiger-mode. In the linear regime, the APD is biased below its breakdown voltage, and the output photocurrent of the APD is proportional to the intensity of light striking the APD absorption region and to the APD gain that occurs in its multiplication region. In the Geiger-mode of operation, the APD is biased above its breakdown voltage. In this mode of operation, a single photon may lead to an avalanche breakdown resulting in a detectable current running through the device, which thereafter remains in a conductive state. Consequently, the amplitude of the output signal in the Geiger-mode is constant and is not proportional to the number of photons absorbed.
U.S. Pat. No. 6,741,341 issued to DeFlumere on May 25, 2004, discloses a laser rangefinder that uses an avalanche photodiode (APD) detector array to determine range to a target. U.S. Pat. No. 5,892,575 issued to Marino on Apr. 6, 1999, discloses a laser rangefinder that uses an avalanche photodiode (APD) detector array to determine range to a target. Neither of these disclosures however, is configured to determine range to a target using a non-imaging device that evenly distributes reflected light from the target due to a single pulse.
The laser rangefinder disclosed by DeFlumere is subject to background noise and must be operated with a very narrow bandpass filter of one nanometer, in order to reduce the background noise. In addition, the disclosed laser rangefinder must be operated together with an IR system that can obtain separate IR image data. Accordingly, the disclosed laser ranger finder is expensive and has a limited capability.
The present invention provides a laser rangefinder that uses an APD detector array operating in a Geiger-mode that does not have the deficiencies of the laser rangefinder disclosed by DeFlumere or Marino.